1. Field of the Invention
The present invention relates to a projection optical system and an image projecting apparatus.
2. Description of the Related Art
In recent years, an enlarging projection system has attracted attention which uses a displaying device (referred to as a light valve, below) in which a transmission-type or reflection-type dot matrix liquid crystal and a DMD (Digital Micro-mirror Device) is used and which enlarges and projects an image displayed on the light valve onto a screen so as to exhibit it as a large image.
However, it is not easy to obtain a large-format liquid crystal displaying device with no defect in the production thereof in order to increase the surface area of a light valve, itself, for projecting a large image, and if it were obtained, it would be extremely expensive.
In such a situation, when a light valve such as a transmission-type (or reflection-type) liquid crystal and a DMD is used and an image displayed thereon is enlarged and projected, the size of a screen is not restricted and it is possible to obtain a powerful large image. Therefore, an image-enlarging projection apparatus (projector) has been widely utilized in an office, a school, or home.
However, when a situation of practical utilization of a projector is assumed, a location for setting a projector may be problematic.
For example, when a front-type projector for projecting an image in front in an office is utilized by a few people in a relatively small conference room, no small restriction is provided on a location for setting a projector due to, for example, the size of a projected image, a projection distance, connection with a PC, a table layout for comfortable discussion, and a problem of less usability thereof may occur. Also, in particular, when a presentation material is projected on a projector and explained, a presenter may be forced to stand between the projector and a screen. Then, a shadow of the presenter may be reflected on the screen and a problem may occur such that an audience cannot see a portion of a projected image.
Recently, various kinds of devices for reducing a projection distance with respect to a large screen have been developed increasingly. However, for example, a projection distance of at least about 1 m is required for a front-type one in order to obtain an image with a diagonal size of 50-60 inches in a conventional technique, and at such a distance, a problem often occurs such that the shadow of a presenter reflects on a screen.
Also, a rear-type displaying device, namely, a rear projector, has bee provided in which a projector is contained in a cabinet and rear projection is made on a screen provided on the front face of the cabinet so as to allow an enlarged image to be seen from the side of front face of the cabinet, and a thin-type one is needed in order to realize reduction of the depth thereof. Herein, even if several times of folding are made with, for example, plane mirrors in a housing, there is a limitation on miniaturization of the device, itself, and therefore, reduction of a projection distance of an optical system is desired increasingly.
Japanese Patent Application Publication No. 2002-040326 discloses a conventional technique for a reflection-type imaging optical system, which aims to provide a reflection-type imaging optical system which can suppress size increasing of an imaging optical system and attain a wide angle of view. As a practical example thereof, a reflection-type imaging optical system with four (first to fourth) reflection mirrors is provided, wherein the first reflection mirror has a concave surface and the second to fourth reflection mirrors are convex ones. Also, at least one reflection mirror of each reflection mirror is formed into a shape of free-form surface so as to ensure the desired projection performance.
Japanese Patent Application Publication No. 2002-174853 discloses a conventional technique relating and being limited to a rear-projection-type display device, in which the projection distance to a screen has been reduced. As a method for reducing the depth of the device, there is provided a conventional technique for reducing the projection distance by, for example, arranging a pair of concave mirrors and a convex mirror having a diverging function in an optical path from a displaying optical unit to a rear-surface reflection mirror in the order of the concave mirrors and the convex mirror from the side of displaying optical unit.
Japanese Examined Patent Application No. H06-091641 discloses a conventional technique relating to a video projector. There are descriptions of a rear-projection-type video projector intended to provide a thin-type one in which the first mirror surface of a television set is formed into a convex shape.
A common problem to the conventional techniques described above is to use many aspheric surfaces including a free-form surface in order to maintain an imaging performance and realize a wide angle of view, and accordingly, it is necessary to keep a strict precision of the distance between respective surfaces as well as a precision of the surface shape. Particularly, since the influence of an error in regard to the precision of a mirror surface is large in a type including the reflection mirrors in Japanese Patent Application Publication No. 2002-040326, there is a problem of setting of a comparatively narrower tolerance among these conventional projection-optical systems.
Also, when composed of only reflection mirrors, there is provided a merit of no generation of a chromatic aberration in principle. On the other hand, it is necessary to insert a color combining prism such as a cross prism and a Philips prism and a chromatic aberration is caused in case of an image forming system which forms a color image by combining plural colors. Therefore, if the projection optical system is composed of only reflection mirrors, there is provided a demerit such that the chromatic aberration cannot be compensated for.
Thus, in regard to the projection optical system, projection-type image displaying device and image displaying system disclosed in Japanese Patent Application Publication No. 2004-061959, a lens system and plural reflective surfaces having a power are used in the projection optical system. Therefore, it is possible to compensate for a chromatic aberration caused by, for example, the color combining prism, by using the lens system. However, 3-5 aspheric mirrors are used in the practical examples and the cost is high. Also, it is necessary to set the surface precision and positional precision of the reflective surfaces to be extremely high similarly to the publicly known case described above, and there is a problem of a strict precision of assembly for the optical system. Furthermore, since light beams emitted from an image forming element is not telecentric, the brightness at an image plane may not be uniform or it may be necessary to make the effective angular range of a film of the color combining prism be wider than the divergence angle of light emitted from the light valve, whereby the separation characteristic may be degraded.
Also, in regard to the projection optical system, enlarging projection optical system, enlarging projection apparatus and image projecting apparatus disclosed in Japanese Patent Application Publication No. 2004-258620, a lens system and plural reflective surfaces having a power are used in the projection optical system. Since enlargement is attained by using one rotationally aspherical reflective surface, the cost is saved and the precision of assembly is totally lowered compared to the technique disclosed in Japanese Patent Application Publication No. 2004-061959. However, since the lens system is parallel decentered and tilted with respect to the optical axis in practical examples 1-5, it is very difficult to conduct the centering thereof. Also, a lens system is used without decentering in practical example 6, but the diameter of a lens that is closest to a screen is large due to the lens configuration, and therefore, the cost is high. That is, since the lens configuration is positive, negative and negative, the diameter of the last lens is large, which is a factor of cost increase.
In such a situation, the inventor(s) considered to provide a more compact projection optical system.
Also, the inventor(s) considered to provide an image projecting apparatus which includes a more compact projection optical system.